1. Field of the Invention
The present invention generally relates to a catalyst heating control apparatus, and more particularly to an apparatus for controlling heating of a catalytic converter of an internal combustion engine in order to achieve an efficient catalytic conversion of exhaust emission of the engine.
2. Description of the Related Art
Generally, a catalytic converter of an internal combustion engine does not achieve an efficient catalytic conversion of exhaust gas from the engine when the catalyst temperature is lower than a catalyst activation temperature (hereinafter called a target temperature). The catalyst of the catalytic converter is heated by the exhaust gas during the engine operation so that the catalyst temperature is increased by the heat of the exhaust gas to the target temperature.
However, when the engine has just started operating under a cold start condition, the engine is not yet warmed up and the engine combustion efficiency is low. The temperature of exhaust gas at such a time is low, and the catalytic conversion efficiency relating to the catalyst subjected to the exhaust gas in the cold start condition becomes very low.
In order to eliminate the above mentioned problem and achieve an efficient catalytic conversion of exhaust gas, it is necessary to predict the temperature of the catalyst from the operating condition of the engine. Japanese Laid-Open Utility Model Publication No. 60-145278 discloses a catalyst heating control device of an internal combustion engine in which the catalyst temperature is predicted from an engine cooling water temperature sensed from the engine. Hereinafter, the temperature of engine cooling water of the engine is called the engine temperature. In the conventional device disclosed in the above mentioned publication, no consideration is given as to the engine temperature at which the engine started operating.
In the conventional device disclosed in the above mentioned publication, if the sensed engine temperature is below a fixed reference temperature, it is determined that the catalyst temperature is below the target temperature. Thus, the catalyst of the catalytic converter at this time is heated by setting up a delayed ignition time condition in which an ignition time relating to a spark plug of the engine is delayed. On the other hand, if the sensed engine temperature is higher than the fixed reference temperature, it is determined that the catalyst temperature is higher than the target temperature. Thus, the catalyst of the catalytic converter at this time is released from the heating due to the ignition timing delay, so that the catalyst is heated by the heat of the exhaust gas.
FIG. 1 shows the operation of the conventional device disclosed in the above mentioned publication. When the engine has started operating at a normal temperature (e.g., 0 deg. C.), the engine temperature indicated by a line Ia in FIG. 1 begins to increase towards a fixed reference temperature THWo (e.g., 50 deg. C.). When the engine temperature reaches the reference temperature THWo, the catalyst temperature indicated by a line IIa in FIG. 1 is actually equal to a target catalyst temperature To (e.g., 300 deg. C.). Thus, if the catalyst of the catalytic converter is released from the heating due to the ignition time delay when the engine temperature (the engine cooling water temperature) reaches the fixed reference temperature THWo, the catalyst is heated by the heat of the exhaust gas in a manner suitable for achieving an efficient catalytic conversion of the exhaust gas.
However, when the engine has started operating at a very low temperature (e.g., -50 deg. C.), it takes a longer time for the engine temperature to rise to the reference temperature THWo. It is difficult to correctly predict the catalyst temperature from the sensed engine temperature (the temperature of engine cooling water). Changes of the engine temperature and the catalyst temperature after the starting of the engine operation are indicated by lines Ib and IIb in the timing chart in FIG. 1. When the engine temperature has reached the reference temperature THWo, the catalyst temperature has become much higher than the target temperature To. Thus, if the catalyst is released from the heating due to the ignition time delay when the engine temperature reaches the fixed reference temperature THWo, it is likely that the catalyst deteriorates or is damaged by the heat excessively supplied thereto in the delayed ignition time condition.
In addition, in the case of the conventional device indicated by the lines Ib and IIb in FIG. 1, there is a problem in that the fuel economy becomes worse since the catalyst is heated for an extended duration before the catalyst is released from the heating. When the engine is operating under the delayed ignition time condition, the output power of the engine becomes smaller than the output power when the engine is operating under a non-delayed ignition time condition. In order to obtain the desired output power of the engine under the delayed ignition time condition, it is necessary to set the throttle valve to a throttle angle greater than the throttle angle set under the non-delayed condition. In addition, it is necessary to inject, under the delayed condition, a greater amount of fuel into the intake passage of the engine than the amount of fuel injected under the non-delayed condition. Therefore, the fuel economy becomes worse when the engine starts operating under a cold start condition. In the timing chart in FIG. 1, "a" and "b" indicate changes of an execution flag (used to allow the heating or non-heating of the catalyst) when the starting engine temperature is normal and when the starting engine temperature is very low.